Reduction of intracellular pH (pHi) by acute reduction of extracellular pH (pHe) leads to hyperthermia sensitization and reduced thermotolerance development. However, cells adapt to chronic exposures to low pHe by re-establishing a near-normal pHi, and are no longer hyperthermia sensitive. We and others have shown that much of the tumor population is below normal pHe (mean of 7.2 vs. 7.4) and that hyperglycemia can acutely and specifically reduce tumor pHe by another 0.2 pH units in both American and Chinese patients. The overall objectives of this program are to test the general hypothesis that the response of tumors to hyperthermia can be enhanced by reducing pHi. If manipulation of pH is to lead to an improvement of the therapeutic ratio by trimodality therapy, strategies must be developed based on fundamental principles which can lead to a rational basis for the design of clinical trials. Five interactive projects are proposed. Project #1 tests the hypothesis that the increased capability for cellular proton expulsion can largely be accounted for by increased activity of one or both of the two proton translocating mechanisms which are thought to be the principal systems active in mammalian cells: a) the amiloride-sensitive sodium-proton (Na+/H+) exchange, and b) the sodium-sensitive chloride-bicarbonate (HCO3-/Cl-) exchange. Project #2 tests the hypothesis that inhibition of the Na+/H+ antiport system, inhibition of the H+/lactate symport or inhibition of HCO3-/Cl- exchange in cells adapted to low pHe will reduce pHi and will sensitize to hyperthermia. Whether the hyperthermia response is enhanced primarily by means of sensitization or primarily by inhibition of thermotolerance development will be evaluated. In addition, the hypothesis that the synthesis of certain heat shock proteins is tightly coupled with thermotolerance development will be tested. Project #3 tests the hypothesis that reduction of pHi will enhance heat-induced potentially lethal damage to cellular structural systems, thereby leading to increased killing. Project #4 tests the hypothesis that regions of chronic acidosis brought about by substrate limitation exist in normal bone marrow, and therefore marrow stem cells and committed precursors may also be sensitized to thermochemotherapy by inhibition of the Na+/H+ antiport or inhibition of lactate transport. The effect of hyperthermia on blood flow in human tumors during hyperthermia, and the correlation between blood flow and interstitial pH are documented in Project #5. Alterations in tumor blood flow result in concomitant alterations in the accessibility of metabolites, the accumulation of lactic acid and other metabolic by-products, and the dissipation of temperature by convective processes. There is a Core component which provides administrative, statistical and technical support (e.g., intracellular pH measurement) for all projects.